This disclosure relates to ion-selective electrodes and, more particularly, to a combination ion-selective electrode with a replaceable ion-sensing membrane.
Ion-selective electrodes can be constructed to comprise semi-permeable membranes that separate two aqueous solutions. One solution is a reference solution having a substantially fixed concentration of ions. The other solution is a sample solution containing the ions to be detected. When the electrode is placed in contact with the sample solution, a potential develops across the membrane in accordance with the Nernst equation. This potential, or voltage, varies as a function of the concentration of the ion being detected in the sample solution.
Semi-permeable membranes suitable for ion-selective electrodes may comprise various materials. For example, certain salts of anions such as bromide, chloride, and sulfide which are highly insoluble in aqueous solution. Salt anion membranes commonly take the form of quasi-amorphous masses or highly compressed polycrystalites. Suitable membranes may also comprise massive monocrystals of rare earth fluorides for sensing fluoride ions, or they may simply be glasses known to be sensitive to, for example, hydrogen, sodium, or potassium ions. The membrane may also be formed of ion exchange material held in an inert, porous substrate. Additionally, polyvinyl chloride (PVC) and epoxy may be suitable membrane materials.
A potentiometric measurement cell comprises two half cells. Ion-selective measurement cells are potentiometric cells where one half cell is the ion-selective electrode (xe2x80x9cISExe2x80x9d) and the other is a reference electrode. ISEs and reference electrodes can be constructed as separate assemblies which are then used together in a test solution to comprise an entire measuring cell. It is more common, however, to construct a single assembly or probe which combines both the reference electrode and the ISE. A probe which combines the reference and ion-selective electrodes is referred to as a combination ISE. Combination ISEs are advantageous in that they are often less expensive than a pair of individual half cell electrodes and the compact geometry of a single probe allows use of the measurement cell in smaller volumes of samples.
The reference electrode usually includes an internal reference element composed of a metal or metal-salt combination, one or two reference electrolyte solutions (salt bridge solutions), and one or two liquid junctions which are small orifices, porous barriers, or other restrictions which inhibit excessive mixing of a reference electrolyte with the sample solution, yet provides electrical contact between the reference and the sample. The ion-selective electrode incorporates a sensing membrane which is permanently sealed into the body of the electrode.
The ion-selective sensing membrane tends to have the shortest useful life of all the measurement cell components. Typically, when the ion-selective sensing membrane has expended its useful life, the membrane cannot be removed from the combination ISE because it is permanently sealed into the electrode body. Thus, the combination ISE cannot be rebuilt and must be discarded. The cost of buying a new electrode is much higher than the cost of replacing just the ion-selective sensing membrane. Moreover, the time involved in procuring a new electrode from the supplier is much greater than the time involved would be in disassembling the old electrode, replacing the membrane, and reassembling the electrode.
A half cell ion-selective electrode with a replaceable sensing membrane is known, but this half cell is not available in a combination ISE. A combination ISE with a replaceable sensing module is also known, but, as with the half-cell apparatus, the replaceable module includes an internal reference electrode element and reference electrolyte contained in chamber of the module. The replaceable module does not provide for replacing only the ion-selective membrane. The replaceable module I SE incidentally provides for replacement of the ion-selective membrane, but only together with the other elements of the module. Replacing the ion-selective membrane in a combination ISE poses different problems from replacing the membrane in an half-cell electrode. In this latter combination ISE module, the module further forms a part of the liquid junction as an annular sleeve disposed within the probe body. This modular design relies on a spring-loaded outer body seating properly on the tapered end of the bottom section of the module to control the flow rate of reference electrolyte flowing at the reference junction. Replacement of the module therefore affects the integrity of the reference junction. It would be desirable that the membrane be removable without causing the reference electrolyte to flow excessively after replacing the membrane in a combination ISE. The replaceable module is directed toward a different problem from that of the present combination ISE, and therefore does not provide a combination ISE that provides for replacement of only the ion-selective membrane. And further, the replaceable module does not provide an apparatus where the membrane replacement assembly is sequestered from the reference electrolyte so that the membrane may be replaced without disturbing the reference electrolyte solution.
The prior art also provides a cap assembly for replacing a selectively gas-permeable membrane in a non-ISE electrode. The cap assembly of this prior art comprises a housing having a port, an internal conical surface sloping away from the port, an annular groove in the conical surface, an annular O-ring type sealing element disposed in the groove, a disk-type sealing element seated on the O-ring, and a membrane securing element having a clamp element that mates with the conical surface of the housing. The assembly is suitable for detecting gases in a gaseous state that diffuse across the membrane, but is not suitable for detecting ions in an analytical sample where the ions do not diffuse across the membrane in a gaseous state. That is, the assembly of this prior art is not suitable for ISEs. One reason this prior art assembly is unsuitable for ISEs is that the assembly does not provide semi-permeable membrane with ion selective materials between the reference electrolyte solution and the analytical solution, and therefore the electrode cannot detect ions in solution in the analytical sample. The membrane itself may allow gas permeation in this prior art. However, it is undesirable for the membrane to allow ion permeation as required in an combination ISE. Another drawback of this prior art is that the cap assembly snaps on and off of the end of the electrode to replace the membrane. To remove the cap assembly for membrane replacement, one must apply torsional or axial force to the electrode body. Application of such forces to the electrode body increases the likelihood that excessive force will be applied, occasionally resulting in the destruction of the electrode body requiring replacement of the entire electrode apparatus at great expense.
The ion-sensing membrane of the combination ISE is the component that has the most exposure to external elements. This continual exposure to the external environment contributes to the short useful life of the membrane. Sample contamination, sample temperature, sample pH, sample pressure, physical abuse, membrane aging, and exposure to ultra-violet and other ambient radiation all contribute to the deterioration of the membrane. However, there is no known combination ISE which allows the user to simply replace the membrane after the membrane has expended its useful life. One must either use a half cell ion-selective electrode with a replaceable sensing membrane together with a separate reference electrode, or one must use a combination ISE where only a module comprising a membrane together with an internal reference element and internal reference electrolyte can be replaced. Those skilled in the art will appreciate the present invention that addresses these and other problems.
Therefore, a combination ISE with a replaceable ion-sensing membrane is provided. The membrane is easily replaced without sacrificing the other components of the combination ISE, achieving cost and time-saving benefits. These benefits are particularly important where continuous or frequent sample analysis is required (for example, industrial processes) and the combination ion-selective electrode may be potted in a suitable industrial housing design. The replaceable membrane provided herein does not form part of the liquid junction or salt bridge with the reference electrolyte; therefore, there is no danger of reference electrolyte leakage upon replacing the membrane. This may be contrasted against known cartridge designs where the membrane must be removed together with a reference electrolyte module and the entire cartridge is replaced on the electrode. Provided is a combination ISE where the ion-selective sensing membrane is the only removable part, and this membrane does not, in any manner, form part of the liquid junction with the reference electrolyte.
The combination ion-selective probes with replaceable sensing membranes provided herein were designed and evaluated for a wide-range of sample analytical ions, including but not limited to ammonium, bromide, cadmium, calcium, chloride, copper, cyanide, fluoride, fluoroborate, iodide, lead, magnesium, nitrate, perchlorate, potassium, silver, sulfide, and surfactant.
Probes were tested for overall performance characteristics equivalent to or better than:
comparable half cell ion-selective electrodes used with separate single or double junction reference electrodes;
overall performance characteristics equivalent to or better than comparable combination ISEs in which the sensing membrane was sealed into the body and not replaceable; and
easily replaceable ISE membranes.
In one embodiment, the combination ISE provided herein comprises a sensing membrane in the form of a replaceable PVC cartridge and sealed into the combination ISE body with one or two O-rings. This replaceable PVC cartridge is designed for, but not limited to, ammonium, calcium, fluoroborate, magnesium, nitrate, perchlorate, potassium, and surfactant.
In another embodiment, the combination ISE comprises a sensing membrane in the form of a pressed pellet encased in a replaceable epoxy cartridge and sealed into the combination ISE with one or two O-rings. This replaceable epoxy cartridge is designed for, but not limited to, the following analytical ions: bromide, cadmium, chloride, copper, cyanide, iodide, lead, silver and sulfide.
In yet another embodiment, the combination ISE comprises a sensing membrane in the form of a solid crystal and is sealed into the combination ion-selective electrode body with one or two O-rings. This replaceable crystal is designed for, but not limited to the analytical ion fluoride.
One advantage of the present device is that when the sensing membrane fails or loses performance for whatever cause, it can be immediately replaced without discarding any other portion of the combination ISE. This results in time and costs savings.
Another advantage is that the membrane replacement has no unfortunate side effects on the performance of the reference electrode, whereas membrane replacement with an annular sleeve module can affect the performance of the reference electrode.
Yet another potential advantage of the present device is the option of interchangeable sensing membranes, where one can select a membrane most suitable for a particular analytical ion. A single combination ISE body can be used to measure a variety of analytical ions by simple replacement of the membrane cartridge or crystal. This offers benefits in multiple ion analysis whereby an electrode kit containing various replaceable membranes can be made available for the analysis of different determinands done on an occasional basis. The known modular designs for ISEs rely on a spring-loaded outer body, seating properly on the tapered end of the bottom section of the module to control the flow rate of the reference electrolyte flowing at the reference junction. In contrast, the combination ISE provided herein does not require any proper seating at the junction with the reference electrolyte. That is, the ion-sensitive membrane provided herein is easily replaceable without endangering the integrity of the reference electrolyte junction.